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Yusufu N. Sulai, Alfredo Dubra; Adaptive Optics Scanning Ophthalmoscopy With Amplitude Pupil Apodization. Invest. Ophthalmol. Vis. Sci. 2012;53(14):5667.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the use of apodization masks in an adaptive optics (AO) scanning light ophthalmoscope for increasing lateral resolution and attenuation of cone photoreceptor image intensity relative to that of rods by exploiting the Stiles-Crawford effect (SCE).
Sequences of 400 to 800 photoreceptor images showing a 0.75° field of view at the retina were acquired at the foveal center and 10° temporal from fixation in two subjects with no eye disease. The images were collected using 0.97 and 0.58 Airy disk diameter confocal apertures and 680 nm light from a superluminescent diode. Circularly symmetric binary masks were centered in the pupil planes to block the central 2, 3 or 4 mm over a 7.8 mm pupil at the eye in the illumination and/or imaging paths of the instrument.
The normalized radial average of the Fourier transform of the photoreceptor images show a relative enhancement of the high spatial frequencies, as predicted by the theory of confocal microscopy modified to incorporate apodization. The width of the central lobe of the autocorrelation of the foveal cone images is reduced by 10-20% when using the 2 and 3 mm masks in the illumination pupil, indicating a narrowing of the average cone photoreceptor image, therefore increasing the ability to resolve them. Both automated and manual counting identified more foveal cones when the illumination pupil was apodized, but not necessarily when the imaging pupil was apodized. The apodization attenuates the intensity of cone relative to rod photoreceptors as expected from the SCE.
Pupil apodization using circular opaque masks in the illumination path of AO scanning instruments can improve our ability to resolve the photoreceptor mosaic in vivo, thus increasing the clinical potential of AO imaging. Comparing images collected with and without this type of apodization mask can be used to distinguish cone from rod photoreceptors, a task usually complicated by the non-uniform appearance of cones away from the foveal center. Given that apodization of the illumination pupil elongates the point spread function axially, this could be exploited to increase the depth of focus in AO optical coherence tomographs, without sacrificing transverse resolution. Finally, centered opaque apodizing masks could be used to attenuate undesired corneal, spectacle and lens reflections in ophthalmic AO scanning instruments.
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